Bringing order to large-scale disordered complex metal alloys: Gd2Au15−xSbx and BaAuxGa12−x

Abstract

Herein, two new tetragonal complex metallic alloys (CMAs) have been discovered and characterized: Gd₂Au_15−xSb_x [x = 3.0–3.6; I4/mmm; tI34; a = 7.31–7.33 Å, c = 14.05–14.11 Å; V = 750.2–758 ų] and BaAu_xGa_12−x [x = 3.6–4.4; I4/mcm; tI104; a = 8.77–8.78 Å, c = 26.06–26.13 Å; V = 2006.5–2010 ų]. Both structures incorporate significant anionic site mixing and intricate positional disorder. Gd₂Au_15−xSb_x represents a new structure type with in-plane disordered, but strongly geometrically restricted rhombi motifs. These rhombi connect through additional mixed Au/Sb positions along the c axis; this forms octahedral fragments. BaAu_xGa_12−x finds its place within the extended NaZn₁₃ structural family, displaying cation-centered snub cubes with empty, distorted icosahedra (allowing for the 1 : 12 ratio) and tetrahedral stars. The split positions order due to geometric constraints to form nets of crown cyclooctane-like sheets in two different conformations. Adjacent planes adopt opposite conformations, forming layers of snub cubes with order between layers, but disorder across the greater structure. The substantial degree of mixing in both structures, together with mutual orientation of the ordered and disordered positions, suggests significant importance of heteroatomic bonding, typically found in polar intermetallic compounds.